1. Field of the Invention
This invention lies in the field of thermally conductive, electrically insulative thermoplastic resins and processes for making the same.
2. Prior Art
In electronic and electrical components and apparatus, due to increasing integration, and packing density, problems with heat removal through plastic envelopes occur. Such problems are found in individual integrated circuits where the heat produced during chip operation must be dissipated (abducted or removed) through the encapsulation means therefor, such as, for examples, encased hybrid circuits, housings of thermally highly loaded apparatus, and the like. An electrically insulating plastic having good heat (thermal) conducting properties is always desired. Such a property combination is also required in other fields of electrical engineering; for example, in small motors.
The thermal conductivity of unfilled, non-foamed plastics lies between approximately 0.2 and 0.5 Wm.sup.-1 K.sup.-1.
In the case of thermosetting plastic molding or sealing compounds, through the addition thereto of approximately 70% quartz, thermal conductivity values of up to about 2 Wm.sup.-1 K.sup.-1 are obtained. With thermoplastic polymers which are characteristically electrically non-conducting (i.e., insulative), but thermally conducting, as a rule, very hard, particulate fillers with good thermal conductance characteristics are seldom employed, since thereby considerable processing problems (abrasion) occur.
If the electrically insulative property is relinquished, however, thermoplastics can be obtained which, as a consequence of their filler (such as aluminum flakes or the like) exhibit a strongly anisotropic thermal conductivity of about 2 Wm.sup.-1 K.sup.-1 or somewhat more.
It is known that ethylene can be polymerized in the presence of iron powder in such a fashion that the individual iron particles are covered with a polymer layer (Fair-information, Hannover 82, Institute for Technical Chemistry of the Technical University, Berlin: Blasensaule als Polymerisations-Reaktor, Polymer/Metall-Werkstoffe). Although this product shows a markedly higher specific electrical volume resistance than a comparable physical mixture of polyethylene containing the same volume-proportion of iron, this resistance is too small for utilization as an electrical insulation medium compared to the iron contents which lead to the commercially desired values of thermal conductivity.